1. Field of the Invention
The present invention relates to a near-field light generator for use in thermally-assisted magnetic recording in which data is written on a recording medium with its coercivity lowered by irradiating the recording medium with near-field light, and to a thermally-assisted magnetic recording head including the near-field light generator.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head unit including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head unit including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider configured to slightly fly above the surface of a recording medium. The slider has a medium facing surface to face the recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, whereby the area is made to increase in temperature and drop in coercivity, and data is written thereon. The area where data is written subsequently falls in temperature and rises in coercivity to increase in thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near the medium facing surface of the slider. The waveguide includes a core through which light propagates, and a cladding provided around the core.
The plasmon generator has a front end face located in the medium facing surface. The front end face generates near-field light. Surface plasmons are excited on the plasmon generator and propagate along the surface of the plasmon generator to reach the front end face. As a result, the surface plasmons concentrate at the front end face, and near-field light is generated from the front end face based on the surface plasmons.
U.S. Patent Application Publication No. 2010/0172220 A1 discloses a near-field light generator including a waveguide and a plasmon generator. In the near-field light generator, the plasmon generator is disposed at a predetermined distance from the core of the waveguide. In the near-field light generator, evanescent light is generated at the surface of the core and surface plasmons are excited on the surface of the plasmon generator through coupling with the evanescent light.
Materials that are typically employed for plasmon generators are metals having high electrical conductivities, such as Au and Ag. However, Au and Ag are relatively soft and have relatively high thermal expansion coefficients. Thus, forming an entire plasmon generator of Au or Ag gives rise to problems as discussed below.
In the process of manufacturing a thermally-assisted magnetic recording head, the medium facing surface is formed by polishing. During polishing, polishing residues of metal materials may grow to cause smears. To remove the smears, the polished surface is slightly etched by, for example, ion beam etching in some cases. If an entire plasmon generator is formed of Au or Ag, which is relatively soft, the polishing and etching mentioned above may cause the front end face of the plasmon generator to be significantly recessed relative to the other parts of the medium facing surface. In such a case, the front end face of the plasmon generator becomes distant from the recording medium, and the heating performance of the plasmon generator is thus degraded.
Part of the energy of light propagating through the core is transformed into heat in the plasmon generator. Part of the energy of near-field light generated by the plasmon generator is also transformed into heat in the plasmon generator. The plasmon generator thus rises in temperature during the operation of the thermally-assisted magnetic recording head. If the entire plasmon generator is formed of Au or Ag, the rise in temperature of the plasmon generator causes the plasmon generator to expand and significantly protrude toward the recording medium. This in turn may cause a protective film covering the medium facing surface to come into contact with the recording medium and thereby damage the recording medium or be broken. When the protective film is broken, the plasmon generator may be damaged by contact with the recording medium or may be corroded by contact with high temperature air.
Further, if the entire plasmon generator is formed of Au or Ag, the temperature rise of the plasmon generator may result in deformation of the plasmon generator due to aggregation. In addition, such a plasmon generator expands when its temperature rises and then contracts when its temperature drops. When the plasmon generator undergoes such a process, the front end face of the plasmon generator may be significantly recessed relative to the other parts of the medium facing surface. In such a case, the heating performance of the plasmon generator is degraded as mentioned above.
For the various reasons described above, a plasmon generator that is formed entirely of Au or Ag has the drawback of being low in reliability. The drawback becomes more noticeable if the front end face of the plasmon generator is large in area.
U.S. Patent Application Publication No. 2010/0172220 A1 discloses a plasmon generator shaped such that the thickness of a portion of the plasmon generator near the front end face decreases toward the front end face. This plasmon generator allows for a reduction in the area of the front end face. U.S. Patent Application Publication No. 2010/0172220 A1 further discloses a structure in which the plasmon generator has a propagation edge or a propagation surface to allow surface plasmons to propagate therethrough, and a groove for receiving at least a portion of the propagation edge or the propagation surface is formed in a top surface of the core having the top surface and a bottom surface. This structure aims at exciting a lot of surface plasmons on the propagation edge or the propagation surface.
The above-described structure, however, has a drawback that the efficiency of excitation of surface plasmons on the plasmon generator suffers a reduction due to the groove. This will now be described in detail. To begin with, we will consider a first cross section which passes through an edge of the groove closest to the light-incidence end of the core and is perpendicular to the direction of travel of the light propagating through the core. Then, a portion of the core that is located closer to the light-incidence end relative to the first cross section will be referred to as the first portion, and another portion of the core that is located farther from the light-incidence end relative to the first cross section will be referred to as the second portion.
Next, we will consider a second cross section which is parallel to the direction of travel of the light propagating through the core and perpendicular to the bottom surface of the core. On the second cross section, the dimension of the core in a direction perpendicular to the bottom surface of the core is defined as thickness. The first portion does not include the groove, whereas the second portion includes the groove. Consequently, the second portion is smaller in thickness than the first portion. Further, the center of the second portion in the thickness direction does not coincide with the center of the first portion in the thickness direction.
Next, we will consider a typical core having no groove and having a constant thickness. When such a typical core is used to excite surface plasmons on the plasmon generator, the greatest efficiency of propagation of light through the core and the greatest efficiency of excitation of surface plasmons on the plasmon generator are achieved when the optical axis of the light incident on the core coincides with the center of the core in the thickness direction.
To allow light to enter the first portion of the core having the groove, the optical axis of the light is typically aligned with the center of the first portion in the thickness direction. This is for the purpose of achieving the greatest efficiency of propagation of the light through the first portion. The light having entered the first portion propagates through the first portion and enters the second portion. As mentioned above, the center of the second portion in the thickness direction does not coincide with the center of the first portion in the thickness direction. Consequently, when the light enters the second portion, its optical axis does not coincide with the center of the second portion in the thickness direction. This results in a reduced efficiency of propagation of the light through the second portion. As a result, the efficiency of excitation of surface plasmons on the plasmon generator is also reduced.